The fast mechanical response is believed to be the cellular basis of the positive feedback mechanism required for the fine tuning process of the hearing organ. We demonstrated that the fast mechanical response of the outer hair cell is membrane-potential dependent. We found membrane-potential dependent movement of gigaohm sealed membrane patches. This appears to be based on the same mechanism as the fast movement of the whole cell. To investigate what physiological signal causes the fast response, we showed that the lateral wall can also be a mechanoreceptor in addition to the stereocilia. We found macroscopic current produced by osmotic stress, attributable to stretch-activated potassium channels.